Method and an apparatus for dynamically reconfiguring a system bus topology

ABSTRACT

A method and an apparatus is presented for configuring a system bus topology dynamically. In a preferred embodiment, the system bus is a Small Computer System Interface (SCSI) bus that connects a “daisy” chain of disk drives. Two types of disk drives are used: single ended (SE) “Ultra” drives capable of 20 MHz operation and LVD (low voltage differential) “Ultra Plus” drives capable of 40 MHz operation. LVD disk drives can also function in the slower SE mode. The first drive in the chain of drives may need to be connected by a cable over three feet long. This introduces signal degradation that is often overcome by introducing redrive circuitry to boost signal quality. This is an expensive solution and a much easier solution is presented: install a jumper between the last drive in the chain and the first drive. However, if LVD bus mode is used, then this jumper solution does not work and the jumper must be removed. Disk drives in a server system are “hot swappable,” which means they can be changed at run time without shutting down the system. A method and an apparatus is provided for dynamically testing for the appropriate mode of bus operation based on the currently installed disk drives and adjusting the jumper setting accordingly.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to an improved system bustopology and, in particular, to a method and an apparatus forconfiguring a bus topology. Still more particularly, the presentinvention provides a method and an apparatus for detecting the type ofhard drives connected to the bus and automatically configuring the busfor improved performance.

2. Description of the Related Art

In a client-server network many clients are constantly making requeststhat require fast responses from the server or servers. Requeststypically require access to information stored in a large datarepository that is spread over many hard disk drives. The performance ofthe hard disk drive subsystem is critical to overall client-serversystem performance.

The SCSI (Small Computer System Interface) expansion bus is a commonlyused bus to connect a chain of disk drives to a computer system. A diskdrive that connects to a chain of disk drives may have differentcapabilities that other disk drives in the chain. For example, for SCSIhard disks, some may use a 20 MHz mode of operation called “Ultra” whileothers are capable of a 40 MHz mode of operation called “Ultra Plus”. Itwould only take one disk drive capable of only “Ultra” performance toslow an entire chain of disk drives down from a 40 MHz bus speed to a 20MHz bus speed.

The use of the SCSI bus and the bus protocols “Ultra” and “Ultra Plus”are for illustrative purposes only. The type of bus and the types ofprotocols on a particular bus may vary, but the central problem remainsto automatically configure the bus system for improved performance.

In a large server facility where there are many machines each supportingmultiple chains of disk drives, there is the need to accommodate changesin configurations quickly and automatically. For example, it is commonto “hot swap” one disk drive for another disk drive, but it would beprohibitively wasteful for a system administrator to inspect all theother drives in the chain to verify they all use the same protocol.

Therefore, it would be advantageous to have an apparatus and a methodthat allows a system bus to automatically detect the type of hard diskdrives connected to a system bus and to automatically reconfigure thesystem bus based on the type of hard disk drives to provide for improvedperformance.

A server machine should also support multiple disk drives on the samechain and, due to physical constraints of supporting many differentchains, allow for a “long” cable to reach the first disk drive. Atypical chain will contain six disk drives and the cable length may needto be longer than 3 feet. The long cable length results in signaldegradation so that it is difficult to operate several disk driveswithout introducing bus errors. One common approach is a add “redrive”circuitry at the end of the cable to boost the signal strength, but thisis relatively expensive solution.

Therefore, it would be advantageous to find a way to maintain signalquality in the presence of many disk drives on the same chain withoutneeding to install redrive circuitry.

SUMMARY OF THE INVENTION

A method and an apparatus is presented for configuring a system bustopology dynamically. In a preferred embodiment, the system bus is aSmall Computer System Interface (SCSI) bus that connects a “daisy” chainof disk drives. Two types of disk drives are used: single ended (SE)“Ultra” drives capable of 20 MHz operation and LVD (low voltagedifferential) “Ultra Plus” drives capable of 40 MHz operation. LVD diskdrives can also function in the slower SE mode.

The first drive in the chain of drives may need to be connected by acable over three feet long. This introduces signal degradation that isoften overcome by introducing redrive circuitry to boost signal quality.This is an expensive solution and this invention introduces a mucheasier solution: install a jumper between the last drive in the chainand the first drive. However, if LVD bus mode is used, then this jumpersolution does not work and the jumper must be removed.

Disk drives in a server system are “hot swappable,” which means they canbe changed at run time without shutting down the system. This inventionprovides a method and an apparatus for dynamically testing for theappropriate mode of bus operation based on the currently installed diskdrives and adjusting the bus topology accordingly. Although thepreferred embodiment deals with a SCSI system bus and SE or LVD diskdrives, the invention, in general, can be applied to any system bus thatrequires dynamic configuration at run time.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asa preferred mode of use, further objectives and advantages thereof, willbest be understood by reference to the following detailed description ofan illustrative embodiment when read in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a pictorial representation of a distributed data processingsystem in which the present invention may be implemented;

FIG. 2 is a block diagram of a data processing system that may beimplemented as a server in which the present invention may beimplemented;

FIG. 3 is a block diagram of a data processing system that may beimplemented as a client in a distributed data processing system;

FIG. 4A is a block diagram of a “daisy chain” of single ended (SE) diskdrives that perform best using termination and a loop topology inaccordance with a preferred embodiment of the present invention;

FIG. 4B is a block diagram of a “daisy chain” of LVD (low voltagedifferential) disk drives that perform best using only a termination inaccordance with a preferred embodiment of the present invention;

FIG. 4C is a block diagram of a “daisy chain” of disk drives that canaccommodate both SE and LVD modes of operations in accordance with apreferred embodiment of the present invention; and

FIG. 5 is a flowchart illustrating selection of the correct mode ofoperation in accordance with a preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference now to the figures, and in particular with reference toFIG. 1, a pictorial representation of a distributed data processingsystem is depicted in which the present invention may be implemented.

Distributed data processing system 100 is a network of computers.Distributed data processing system 100 contains network 102, which isthe medium used to provide communications links between various devicesand computers connected within distributed data processing system 100.Network 102 may include permanent connections, such as wire or fiberoptic cables, or temporary connections made through telephoneconnections.

In the depicted example, servers 104, 114, 116 and 118 are connected tonetwork 102. Storage units 106 and 122 are also connected to network102, providing backup support for any or all of servers 104, 114, 116and 118. Storage unit 122 provides dedicated backup support for server104. In addition, clients 108, 110 and 112 are also connected to network102. These three clients may be, for example, personal computers ornetwork computers. For purposes of this application, a network computeris any computer coupled to a network, which receives a program or otherapplication from another computer coupled to the network. Distributeddata processing system 100 may include additional servers, clients, andother devices not shown.

In the depicted example, servers 104, 114, 116 and 118 provide storagefor data from clients 108, 110 and 112. These four servers also providedata, such as boot files, operating system images, and applications toclients 108, 110 and 112. Clients 108, 110 and 112 are clients to one orall of servers 104, 114, 116 and 118. Support for a particularapplication being performed on one of clients 108, 110 and 112 may be byone of servers 104, 114, 116 and 118. Additionally servers 104, 114, 116and 118 may provide backup support for each other. In the event of aserver failure, a redundant backup server may be allocated by thenetwork administrator, in which case requests directed to the failedserver are routed to the redundant backup server.

In a similar manner, data backup support is provided by storage units106 and 122 for servers 104, 114, 116 and 118. It is also possible foran individual server to have a data storage unit, such as storage unit120 attached to server 104. Storage unit 120 may be a hard disksubsystem, such as that found in the present invention, with diskschained together to form a “daisy chain” and with one or more daisychains attached to server 104.

In the depicted example, distributed data processing system 100 may bethe Internet, with network 102 representing a worldwide collection ofnetworks and gateways that use the TCP/IP suite of protocols tocommunicate with one another. At the heart of the Internet is a backboneof high-speed data communication lines between major nodes or hostcomputers consisting of thousands of commercial, government, education,and other computer systems that route data and messages. Of course,distributed data processing system 100 also may be implemented as anumber of different types of networks, such as, for example, an intranetor a local area network.

FIG. 1 is intended as an example and not as an architectural limitationfor the processes of the present invention.

Referring to FIG. 2, a block diagram of a data processing system whichmay be implemented as a server, such as server 104 in FIG. 1. Dataprocessing system 200 may be a symmetric multiprocessor (SMP) systemincluding a plurality of processors 202 and 204 connected to system bus206. Alternatively, a single processor system may be employed. Alsoconnected to system bus 206 is memory controller/cache 208, whichprovides an interface to local memory 209. I/O bus bridge 210 isconnected to system bus 206 and provides an interface to I/O bus 212.Memory controller/cache 208 and I/O bus bridge 210 may be integrated asdepicted.

Peripheral component interconnect (PCI) bus bridge 214 connected to I/Obus 212 provides an interface to PCI local bus 216. Network adapter 220is connected to PCI bus 216, but as one of ordinary skill in the artwill appreciate, many other devices can be connected to PCI bus 216.Typical PCI bus implementations will support four PCI expansion slots oradd-in connectors. Communications links to network computers 108-112 inFIG. 1 may be provided through network adapter 220 connected to PCIlocal bus 216 through add-in boards.

Additional PCI bus bridges 222 and 224 provide interfaces for additionalPCI buses 226 and 228, from which additional modems or network adaptersmay be supported. For example, memory mapped graphics adapter 230 isconnected to PCI bus 226. In this manner, server 200 allows connectionsto multiple network computers.

Server 200 will often support substantial data storage, such as server104 supports storage 120 in FIG. 1. This is commonly accomplished byattaching a “daisy chain” of hard disk drives onto a SCSI data bus. SCSIhost bus adapter 240 is connected to PCI bus 216. Hard Disk #1 242 andHard Disk #2 244 are shown connect to SCSI host bus adapter 240, but asone of ordinary skill in the art will appreciate, additional hard diskscan be added, as needed. A preferred embodiment of the present inventiondeals with configuration of a SCSI bus to enhance performance of a setof disk drives on a server machine.

Those of ordinary skill in the art will appreciate that the hardwaredepicted in FIG. 2 may vary. For example, other peripheral devices, suchas optical disk drives and the like, also may be used in addition to orin place of the hardware depicted. The depicted example is not meant toimply architectural limitations with respect to the present invention.

The data processing system depicted in FIG. 2 may be, for example, anIBM RISC/System 6000, a product of International Business MachinesCorporation in Armonk, N.Y., running the Advanced Interactive Executive(AIX) operating system.

With reference now to FIG. 3, a block diagram of a data processingsystem in which the present invention may be implemented is illustrated.Data processing system 300 is an example of a client computer. Dataprocessing system 300 employs a peripheral component interconnect (PCI)local bus architecture. Although the depicted example employs a PCI bus,other bus architectures, such as Micro Channel and ISA, may be used.

Processor 302 and main memory 304 are connected to PCI local bus 306through PCI bridge 308. PCI bridge 308 may also include an integratedmemory controller and cache memory for processor 302. Additionalconnections to PCI local bus 306 may be made through direct componentinterconnection or through add-in boards. In the depicted example, localarea network (LAN) adapter 310, SCSI host bus adapter 312, and expansionbus interface 314 are connected to PCI local bus 306 by direct componentconnection. In contrast, audio adapter 316, graphics adapter 318, andaudio/video adapter (A/V) 319 are connected to PCI local bus 306 byadd-in boards inserted into expansion slots. Expansion bus interface 314provides a connection for a keyboard and mouse adapter 320, and modem322.

In the depicted example, SCSI host bus adapter 312 provides a connectionfor hard disk drive 326, tape drive 328, CD-ROM drive 330, and digitalvideo disc read only memory drive (DVD-ROM) 332. If the client machineis also acting as a “server” for a local area network, it is possiblefor SCSI host bus adapter 312 to act as a data repository for the localarea network. In this case, a daisy chain of hard disk drives can beattached to SCSI host bus adapter 312. For this situation the busconfiguration apparatus and method described in this invention couldalso be applied to Client 300. However, for simplicity, we will refer tothe present invention being installed on a server machine, such asserver 104 in FIG. 1 and the expansion of a server machine in FIG. 2.

An operating system runs on processor 302 and is used to coordinate andprovide control of various components within data processing system 300in FIG. 3. The operating system may be a commercially availableoperating system, such as OS/2, which is available from InternationalBusiness Machines Corporation. “OS/2” is a trademark of InternationalBusiness Machines Corporation.

Those of ordinary skill in the art will appreciate that the hardware inFIG. 3 may vary depending on the implementation. For example, otherperipheral devices, such as optical disk drives and the like, may beused in addition to or in place of the hardware depicted in FIG. 3. Thedepicted example is not meant to imply architectural limitations withrespect to the present invention. For example, the processes of thepresent invention may be applied to multiprocessor data processingsystems.

With reference now to FIG. 4A, a block diagram depicts a “daisy chain”of disk drives that perform best using a SE (single ended) mode, inaccordance with a preferred embodiment of the present invention. Inparticular, for this preferred embodiment, we assume a 20 MHz “Ultra”protocol on the SCSI bus to access the disk drives.

SCSI Controller Host End 402 is attached serially to the first drive inthe chain, Hard Drive #1 (HD1) 406 using “long” SCSI cable 404. In apreferred embodiment, the cable length is 44 inches. Five other harddrives, 408, 410, 412, 414, and 416, are “daisy chained” off of HD1 406.Hard Drive #6 (HD6) connects to Terminator 418. As described in the nextparagraph, there is a performance problem if HD6 is simply terminated.One way to solve this problem is to provide redrive circuitry at the endof the cable, but this is an expensive solution. This inventionintroduces a much easier solution, the installation of jumper 418between HD6 416 and HD1 406 forming a loop topology.

The need for the loop topology is based on the following phenomenon. Ifjumper 418 is not present, an ACK (acknowledge) signal in SE moderesults in “slope reversals” at the near end of the chain (specificallyat HD1 406 and HD2 408). In particular, ACK@HD1 has a deep and longslope reversal that results in SCSI bus errors. This reversal is due toa discontinuity caused by the heavily loaded section of the net withinthe backplane with respect to SCSI cable 402. The large length of thatsection exaggerates the near-end effect for HD1 which results in theobserved deep and long slope reversal. Removal of some of the harddrives from the daisy chain reduces the slope reversal, but this is notan acceptable solution. An alternative solution is to connect thenear-end (HD1) and far-end (HD6) points together that results in removalof the slope reversal and good signal quality on the ACK signal.

With reference now to FIG. 4B, a block diagram depicts a “daisy chain”of disk drives that perform best using a low voltage differential (LVD)mode in accordance with a preferred embodiment of the present invention.In particular, for this preferred embodiment, we assume a 40 MHz “UltraPlus” protocol on the SCSI bus to access the disk drives. Any LVD diskdrive can also function in SE mode, if required.

SCSI Controller Host End 422 is attached serially to the first drive inthe chain, HD1 426 using SCSI cable 424. Five other LVD hard drives,428, 430, 432, 434, and 436, are “daisy chained” off of HD1 426. Thischain of LVD disk drives will produce the best performance if HD6 436 isterminated by Terminator 438. It is important to note that the looptopology shown in FIG. 4A completely destroys the signal quality in LVDmode, so there appears to be no bus configuration compatible with bothSE and LVD protocols.

A key feature of this invention is shown in FIG. 4C, a block diagram ofa “daisy chain” of disk drives that can accommodate both SE and LVDmodes of operation in accordance with a preferred embodiment of thepresent invention. This invention takes advantage of the fact that a LVDdisk drive can also function in SE mode. If any drive in the daisy chainrequires operation in the SE mode, then any other drive in the chainattempting to use LVD mode is switched to SE mode and the entire chainoperates in SE mode. However, if all drives are capable of running inthe faster LVD mode, then the entire chain operates in this mode.

SCSI Controller Host End 442 is attached serially to the first drive inthe chain, HD1 446 using SCSI cable 444. Five other hard drives, 448,450, 452, 454, and 456, are “daisy chained” off of HD1 446. HD6 isconnected to Terminator 460, similar to the configurations shown inFIGS. 4A and 4B. These hard drives may be capable of SE mode only or maybe capable of either SE or LVD modes. HD6 456 is connected to the inputof switch 458 by connector 464. Switch 458 is connected to HD1 446 byconnection 466. The position of switch 458 is determined by the inputsignal 462 named DIFF_SENSE.

In a preferred embodiment, switch 458 is an electro-mechanical relay. Ifswitch 458 receives no voltage (0 volt) at DIFF_SENSE 462, then switchinput from connection 464 is feed to switch output at connection 468 toform a loop topology appropriate for SE mode of operation. If switch 458receives a positive voltage (1 volt) at DIFF_SENSE 462, then switchinput is left open to form a terminated chain appropriate for LVD modeof operation. As one of ordinary skill in the art will appreciate, useof an electronic switch in place of a relay is also possible resultingin an alternative embodiment of the present invention that is lessexpensive and more reliable.

FIG. 5 is a flowchart illustrating the determination of the appropriatesetting for the DIFF_SENSE signal 462 for a preferred embodiment of theinvention. The signal DIFF_SENSE is formed by a “wire AND” of individualsignals from the hard drives (step 502). If any one of these signals islow due to the presence of an SE drive, then the DIFF_SENSE signal islow (step 504: Yes) and the switch is closed to form a loop topology(step 506). If all of the signals are high, then the DIFF_SENSE signalis high (step 504: No) and the switch is open to set the bus to LVD mode(step 508).

The description of the present invention has been presented for purposesof illustration and description but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the art. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

1. A method for configuring a system bus in a data processing system,the method comprising the steps of: detecting an operating mode for eachof a plurality of devices connected to the system bus; identifying amost restrictive operating mode from the detected operating modes; andsetting a bus configuration mode based on the most restrictive operatingmode, wherein setting a bus configuration mode includes setting a switchto one of two states: closed to form a loop configuration and open todisconnect the loop configuration, wherein the plurality of devices aredisk drives connected in a daisy chain of disk drives, wherein theoperating mode of the plurality of disk drives is one of a Single Ended(SE) bus configuration mode or a Low Voltage Difference (LVD) busconfiguration mode, and wherein, if the detected modes include both theSE bus configuration mode and the LVD bus configuration mode, then themost restrictive operating mode is the SE bus configuration mode.
 2. Themethod of claim 1, wherein the system bus is a Small Computer SystemInterface (SCSI) bus.
 3. The method of claim 1, wherein setting the busconfiguration includes enabling a jumper from a last disk drive to afirst disk drive in the daisy chain of disk drives to form a loopconfiguration.
 4. The method of claim 3, wherein the SE busconfiguration mode involves enabling the jumper to form a loopconfiguration.
 5. The method of claim 1, wherein setting the busconfiguration includes disabling a jumper from a last disk drive to afirst disk drive in the daisy chain of disk drives to prevent a loopconfiguration.
 6. The method of claim 5, wherein the LVD busconfiguration mode involves disabling the jumper to prevent the loopconfiguration.
 7. The method of claim 1, wherein indicator signals onthe disk drives are set to a positive voltage if the disk drives operatein the LVD bus configuration mode and are set to zero voltage if thedisk drives operate in the SE bus configuration mode.
 8. The method ofclaim 7, wherein detecting a plurality of devices connected to thesystem bus includes receiving a control signal generated by a logicalAND of the indicator signals from the plurality of disk drives.
 9. Themethod of claim 8, wherein the control signal is zero volts if anyindicator signal from the indicator signals is zero volts and thecontrol signal is a positive voltage only if all of the indicatorsignals are a positive voltage.
 10. The method of claim 8, whereinsetting a bus configuration includes setting a switch to one of twostates: closed to form a loop configuration required for an SE busconfiguration mode and open to disconnect the loop configuration, asrequired by an LVD bus configuration mode.
 11. The method of claim 10,wherein the control signal closes the switch if the control signal iszero volts and the control signal opens the switch if the control signalis a positive voltage.
 12. The method of claim 11, wherein the switch isan electromagnetic relay.
 13. The method of claim 11, wherein the switchis an electronic switch.
 14. A method of configuring a system bus towhich a plurality of devices are coupled, the devices being operated inone of a plurality of modes, the method comprising the steps of:detecting a most restrictive operating mode from the plurality ofdevices; and setting a system bus configuration to be compatible withthe most restrictive operating mode, wherein setting a system busconfiguration includes setting a switch to one of two states: closed toform a loop configuration and open to disconnect the loop configuration,wherein the plurality of devices are disk drives connected in a daisychain of disk drives, wherein the operating mode of the plurality ofdisk drives is one of a Single Ended (SE) bus configuration mode or aLow Voltage Difference (LVD) bus configuration mode, and wherein, if thedetected modes include both the SE bus configuration mode and the LVDbus configuration mode, then the most restrictive operating mode is theSE bus configuration mode.
 15. An apparatus for dynamically configuringa system bus in a data processing system, the apparatus comprising: adetecting means for detecting an operating mode for each of a pluralityof devices connected to the system bus; an identifying means foridentifying a most restrictive operating mode from the detectedoperating modes; and a setting means for setting a bus configurationmode based on the most restrictive operating mode, wherein the settingmeans for a bus configuration mode is a switch with one of two states:closed to form a loop configuration and open to disconnect the loopconfiguration, wherein the plurality of devices are disk drivesconnected in a daisy chain of disk drives, wherein the operating mode ofthe plurality of disk drives is one of a Single Ended (SE) busconfiguration mode or a Low Voltage Difference (LVD) bus configurationmode, and wherein, if the detecting means detects both the SE busconfiguration mode and the LVD bus configuration mode, then the mostrestrictive operating mode is the SE bus configuration mode.
 16. Theapparatus of claim 15, wherein the system bus is a Small Computer SystemInterface (SCSI) bus.
 17. The apparatus of claim 15, wherein the settingmeans for the bus configuration enables a jumper from a last disk driveto a first disk drive in the daisy chain of disk drives to form a loopconfiguration.
 18. The apparatus of claim 17, wherein the SE busconfiguration mode involves enabling the setting means to form a loopconfiguration.
 19. The apparatus of claim 15, wherein the setting meansfor the bus configuration disables a jumper from a last disk drive to afirst disk drive in the daisy chain of disk drives to prevent a loopconfiguration.
 20. The method of claim 19, wherein the LVD busconfiguration mode involves disabling the setting means to prevent theloop configuration.
 21. The apparatus of claim 15, wherein indicatorsignals on the disk drives are set to a positive voltage if the diskdrives operate in the LVD bus configuration mode and are set to zerovoltage if the disk drives operate in the SE bus configuration mode. 22.The apparatus of claim 21, wherein the identifying means for identifyingthe most restrictive operating mode is a control signal based on alogical AND of the indicator signals from the plurality of disk drives.23. The apparatus of claim 22, wherein the control signal is zero voltsif any indicator signal from the indicator signals is zero volts and thecontrol signal is a positive voltage only if all of the indicatorsignals are a positive voltage.
 24. The apparatus of claim 23, whereinthe control signal closes the switch if the control signal is zero voltsand the control signal opens the switch if the control signal is apositive voltage.
 25. The apparatus of claim 23, wherein the switch isan electromagnetic relay.
 26. The apparatus of claim 23, wherein theswitch is an electronic switch.
 27. An apparatus for dynamicallyconfiguring a system bus in a data processing system, the apparatuscomprising: an indicator signal for identifying an operating mode foreach of a plurality of disk drives connected to the system bus; alogical circuit for identifying a most restrictive operating mode fromthe detected operating modes; and a switch for setting a busconfiguration based on the most restrictive operating mode, wherein theswitch for setting a bus configuration has one of two states: closed toform a loop configuration and open to disconnect the loop configuration,wherein the operating mode of the plurality of disk drives is one of aSingle Ended (SE) bus configuration mode or a Low Voltage Difference(LVD) configuration mode, and wherein the most restrictive operatingmode is the SE bus configuration mode.
 28. The apparatus of claim 27,wherein the system bus is a Small Computer System Interface (SCSI) busand the disk drives connected to the system bus form a daisy chain. 29.The apparatus of claim 27, wherein the switch enables a jumper from alast disk drive to a first disk drive in the daisy chain of disk drivesto form a loop configuration in the SE bus configuration mode.
 30. Theapparatus of claim 27, wherein the switch disables a jumper from a lastdisk drive to a first disk drive in the daisy chain of disk drives toprevent a loop configuration in the LVD bus configuration mode.
 31. Theapparatus of claim 27, wherein the indicator signal on the disk drive isset to a positive voltage if the disk drive operates in the LVD busconfiguration mode and is set to zero voltage if the disk drive operatesin the SE bus configuration.
 32. The apparatus of claim 31, wherein thelogical circuit for identifying a most restrictive operating mode is alogical AND of the indicator signals from the plurality of disk drivesand, wherein the logical circuit generates a control signal that is zerovolts if any indicator signal from the plurality of disk drives is zerovolts and the control signal is a positive voltage only if all of theindicator signals are positive voltage.
 33. The apparatus of claim 32,wherein the control signal closes the switch if the control signal iszero volts and the control signal opens the switch if the control signalis a positive voltage.